The present invention relates to an ink jet printing or recording apparatus in which at least one nozzle ejects a jet of ink subjected to supersonic vibration and a charging electrode selectively charges the ink at a position where the jet separates into droplets whereupon deflecting electrodes deflect the charged droplets of ink causing them to impinge on a recording sheet.
Known ink jet recording apparatus of the type described may be classified generally into a two-value deflection control apparatus, a multi-value deflection control apparatus and a combined apparatus of the two mentioned. In the first or two-value apparatus, ink droplets for printing data are charged (or charged to a high level) while those which are not used for printing are left non-charged (or charged to a low level or to the opposite polarity) so that the recording droplets may be deflected to a large extent by a deflecting electric field to impinge on a recording sheet and the non-recording droplets may be captured by a gutter. Conversely, the non-recording ink may be deflected to a large extent to be captured by a gutter. In this type of apparatus, one nozzle is used for one picture element during the recording operation. In the second or multi-value apparatus, one nozzle is used for three or more picture elements (e.g. 5 mm and 40 dots, assuming 8 dots/mm) and recording droplets of ink are charged to three or more levels (e.g. 40 levels) to be deflected along three or more paths (e.g. 40 paths). In the third or combined apparatus, recording ink droplets are charged in the same way as in the multi-value process. However, this last-mentioned apparatus first deflects recording charged droplets using a deflecting electric field extending in the Y-axis direction so as to cause them to miss a gutter and then deflects them using another electric field in the X-axis direction in accordance with their charging levels, thereby printing out data in the X direction on a recording sheet with positional variations.
Meanwhile, ink to be ejected from a nozzle may be vibrated by any of three known systems: one which imparts pressure oscillation to the ink proper, one which imparts vibration in an intended direction of ink ejection to a nozzle plate having at least one ink ejection hole, and one which applies vibration bodily to an ink ejection head in an intended direction of ink ejection. The first system permits the use of a single nozzle plate having one ejection hole which is bonded to the leading end of a cylindrical electrostrictive vibrator, the other end of which is communicated with a pressurized ink supply box. It also permits the use of a nozzle plate having numerous ink ejection holes which is bonded to the front wall of a pressurized ink supply box in such a manner as to cover a slit provided to said wall of the ink supply box. One or more flat electrostrictive vibrators are mounted on one side wall of the box to impart vibrating pressure to ink inside the box. The second system employs a multi-apertured nozzle plate rigidly mounted to a pressurized ink supply box through an elastic member which is caused to vibrate by an electrostrictive vibrator. The third system drives a head bodily for oscillation by means of a motor, a solenoid device, an electrostrictive vibrator or the like.
In a recording apparatus of the type described, should the formation of ink droplets and the supply of charging voltages (in the form of pulses) to charging electrodes become out of synchronization, ink droplets would not receive the expected amounts of charge and this would result their dislocation on a recording sheet and therefore distort the image being reproduced on the recording sheet. To solve this problem, it has been proposed and put to practice to search for a proper charging phase for the ink droplets and thereby properly determine a timing for applying a charging voltage to a charging electrode as disclosed in Japanese Patent Publication No. 47-43450 and Japanese Patent Application No. 50-60131 which was laid open to public inspection. However, such a system fails in detecting whether or not the amount of deflection is proper although succeeding in determining whether or not the charge is appropriate. If the charging timing is proper, an excessive or deficient amount of deflection will contract or expand the resultant image and still cause distortion of the reproduced image in view of the feed pitch or speed of the sheet which is usually constant.
Generally, an amount of deflection x.sub.d of ink droplets can be expressed as: ##EQU1## where K denotes a constant which depends on the deflecting electrodes, Q.sub.j an amount of charge of the ink droplets, m.sub.j a mass of the ink droplets, v.sub.dp a deflecting voltage, S.sub.dp a spacing between opposite deflecting electrodes and v.sub.j an ejection velocity of the ink droplets.
It will be seen from Eq. (1) that the amount of deflection x.sub.d depends on so many factors that mere detection of the charge amount cannot determine the deflection amount and therefore cannot contribute to adequate deflection control. Even though the factors K, V.sub.dp and S.sub.dp in Eq. (1) may be constant (in fact, they can be maintained constant relatively easily) and the charge amount may be controlled to a proper value by a phase search, the factors m.sub.j and v.sub.j still exist as fluctuant factors. More specifically, those factors v.sub.j and m.sub.j are susceptible to variations in the viscosity of ink resulting from temperature variations of the ink and, also, to a slight variation in the property of ink from a state after storage for a long time to a state just after the replacement of the ink. It is therefore, desired to determine a proper amount of deflection by detecting an amount of deflection through the search for a droplet forming phase, rather than an optimum charging timing.